Download Objects

Generic Programming
Amit Shabtay
The Problem

Assume we have a nice Stack implementation.

The problem:
• Our stack receives only Objects
• We need different stacks for int, String and
Method.
• We don’t want the problems of:
Stack.add(“clearly not a method”);
…
Method m = (Method)stack.pop();
March 3rd, 2004
Object Oriented Design Course
2
Solution (?)



Let’s create IntStack , StringStack
and MethodStack.
All of them will rely on the original
Stack as internal implementation.
Are there any problems?
• A lot of code duplication
• It’s hard to add new types
March 3rd, 2004
Object Oriented Design Course
3
Another Problem


We want to use a swap function between
two ints.
void swap(int& a, int&b) {
int temp = a; a = b; b = temp;
}
What about swap(double&, double&) and
swap(Method&, Method&) ?
March 3rd, 2004
Object Oriented Design Course
4
The Actual Solution

Generic programming.
(The ability to have type parameters on your type)

Write the code once, and worry about
the type at compile time
• The code is suitable to all types
• Easy to add types later
• No code duplication
• Demands from types
March 3rd, 2004
Object Oriented Design Course
5
So How Do We Do It?
swap<Class T>(T& a, T& b) {
T temp = a;
a = b;
b = temp;
}
 Looks simple?
March 3rd, 2004
Object Oriented Design Course
6
Java 1.4.2 vs. 1.5 and Autoboxing



ArrayList list = new ArrayList(); //1.4.2
list.add(0, new Integer(42));
int total = ((Integer)list.get(0)).intValue();
ArrayList<Integer> list = new ArrayList<Integer>();//1.5
list.add(0, new Integer(42));
int total = list.get(0).intValue();
ArrayList<Integer> list =
new ArrayList<Integer>(); //1.5 auto-boxing
list.add(0, 42);
int total = list.get(0);
March 3rd, 2004
Object Oriented Design Course
7
Uses

Containers

Algorithms
• list
• set
• vector
• map
• sort
• search
• copy
March 3rd, 2004
Object Oriented Design Course
8
C++ Templates


The most known use of generic
programming
STL – Standard Template Library
• Containers
• vector, set, hash_map
• Algorithms
• for_each, swap, binary_search, min, max
March 3rd, 2004
Object Oriented Design Course
9
What about Java?


Until now Java had large collection set
• Set, List, Map, Iterator, and more
• sort(), search(), fill(), copy(), max(), min()
One major problem – the collections are
not type safe
• No problem to do
Map.put(“key”, “4”);
Integer i = (Integer)map.get(“key”);
March 3rd, 2004
Object Oriented Design Course
10
Java Generics


Added as one of the new features of
Java 1.5 (“Tiger”)
Done in the compiler only
• Converts
String s = vector<String>.get(3) to
String s = (String)vector.get(3)
March 3rd, 2004
Object Oriented Design Course
11
How to Use Generics ?
List<Integer> myIntList = new LinkedList<Integer>();
myIntList.add(new Integer(0));
Integer x = myIntList.iterator().next();
March 3rd, 2004
Object Oriented Design Course
12
And What About the Collection ?
public interface List<E> {
void add(E x);
Iterator<E> iterator();
}
public interface Iterator<E> {
E next();
boolean hasNext();
}
March 3rd, 2004
Object Oriented Design Course
13
Subtyping
List<String> ls =
new ArrayList<String>();
List<Object> lo = ls; //Compiler will not permit
lo.add(new Object());
//Attempts to assign an Object to a String!
String s = ls.get(0);
March 3rd, 2004
Object Oriented Design Course
14
Subtyping (cont.)



Foo is subtype of Bar if:
• Foo extends Bar
• Foo implements Bar
C is a generic container C<E>
Results that C<Foo> is not subtype of
C<Bar>
March 3rd, 2004
Object Oriented Design Course
15
Generic Algorithms (1)
How to print entire Collection?
 Do we have to use Collection<Object> ?
 Use wildcard
void printCollection(Collection<?> c){
for (Object e : c) {
System.out.println(e);
}
}

March 3rd, 2004
Object Oriented Design Course
16
Generic Algorithms (2)

What happens when we want to use specific
method?
public void
drawAll(List<Shape> shapes) {
for (Shape s: shapes) {
s.draw(this);
}
}

What about subtyping?
• List<Circle>
March 3rd, 2004
Object Oriented Design Course
17
Generic Algorithms (3)

The solution
public void
drawAll(List<? extends Shape> shapes)
{…} //Called bounded wildcard.
March 3rd, 2004
Object Oriented Design Course
18
More About Wildcards
Collection<?> c = new ArrayList<String>();
c.add(new Object());
public void addRectangle(List<? extends
Shape> shapes) {
shapes.add(0, new Rectangle());
}
March 3rd, 2004
Object Oriented Design Course
19
Super vs. Extends


The syntax ? super T denotes an unknown
type that is a supertype of T.
It is the dual of the ? extends T to denote an
unknown type that is a subtype of T.
March 3rd, 2004
Object Oriented Design Course
20
Collection<Object>
vs Collection<?> vs Collection



Collection<Object> is a collection of
heterogeneous instances of potentially no
common type
Collection<?> is a collection of homogeneous
instances of some common types – we just
don’t know what that common type is
Collection is a raw type – we should avoid it
March 3rd, 2004
Object Oriented Design Course
21
Many More Features



Java Generics are one of the important
language features of Java 1.5
More information in
http://java.sun.com/developer/technic
alArticles/J2SE/generics/
J2SE 5.0 in a Nutshell
http://java.sun.com/developer/technic
alArticles/releases/j2se15/
March 3rd, 2004
Object Oriented Design Course
22
Java Generics Summary


Java Generics use a technique known as
type erasure which is the process of
translating or rewriting code that uses
generics into non-generic code
all information between angle brackets
is erased.
March 3rd, 2004
Object Oriented Design Course
23
C# Generics




Very similar to Java
public struct Point<T>
{ public T X; public T Y; }
Point<int> point;
point.X = 1; point.Y = 2;
See http://msdn.microsoft.com/
for more information
March 3rd, 2004
Object Oriented Design Course
24
Java Generics vs. C++ Templates vs. C#



Java borrowed C++ syntax and made it
mean something very different
Type erasure vs. code generation in
C++.
Evaluated in compile time in java and
c++, run time in c#
March 3rd, 2004
Object Oriented Design Course
25